Unsymmetrical Thienopentalenes: Synthesis, Optoelectronic Properties, and (Anti)aromaticity Analysis

The synthesis and properties of a series of unsymmetrical thienopentalenes are explored, including both monoareno and diareno derivatives. For the synthesis of monoareno pentalenes, a carbopalladation cascade reaction between alkynes and gem-dibromoolefins was applied. Diareno pentalene derivatives were accessed via gold-catalyzed cyclization of diynes. Thiophene was fused to pentalene in two different geometries via its 2,3 and 3,4 bonds. 2,3-Fusion resulted in increased antiaromaticity of the pentalene unit compared to the 3,4-fusion both in the monoareno and diareno framework. Monothienopentalenes that contained the destabilizing 2,3-fusion could not be isolated. For diareno derivatives, the aromatic character of the different aryl groups fused to the pentalene was not independent. Destabilizing fusion on one side resulted in alleviated aromaticity on the other side and vice versa. The synthesized molecules were characterized experimentally by 1H NMR and UV–vis spectroscopies, cyclic voltammetry, and X-ray crystallography, and their aromatic character was assessed using magnetic (NICS and ACID) and electronic indices (MCI and FLU).

The maximum and minimum residual electron density in the final difference map was 0.69 and -0.25e.Å -3 .
Hydrogen atomic positions were calculated from assumed geometries. Hydrogen atoms were included in structure factor calculations but they were not refined. The isotropic displacement parameters of the hydrogen atoms were approximated from the U(eq) value of the atom they were bonded to. The structure is presented using the program Mercury. 4 CCDC 2090075 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

S1.1 Comparison of solid-state packings of VI and DBP
In the crystal of compound VI, one molecule with a disordered phenyl substituent was found in the asymmetric unit in the space group P2 1 /c ( Figure S3). Figure S3. The asymmetric unit of the crystal of compound VI.
For DBP, 5 four molecules with disordered phenyl groups were found in the asymmetric unit in the space group P-1 ( Figure S4). Figure S4. The asymmetric unit of the crystal of DBP.

S9
DBP has similar supramolecular interactions within the crystal as compound VI despite the number of molecules in the asymmetric unit. The disordered phenyl groups within the molecules are present in two positions that are perpendicular to each other (83(1)⁰ and 85(5)⁰ in VI and DBP, respectively, relative to the plane of conjugated cores).
The conjugated cores of both molecules are arranged parallel due to π-stacking with a slight displacement along their longer axis ( Figure S5). Figure S5. π-Stacked dimers of VI and DBP. One disordered position is omitted for clarity.
Besides the parallel π-stacking, "T-shaped" π-π interactions are present within the crystal lattices, leading to a herringbone arrangement in both cases ( Figure S6). of compounds III and V(PMP). Spectra were recorded in CHCl 3 .
The solution and the electrodes were housed in a 4 mL screw cap vial. The following electrodes were used for all measurements ( Figure S8): Pt counter-electrode (CE), Pt wire workingelectrode (WE), Ag/AgCl reference electrode.   Figure S9). Electrochemical data is summarized in Table S3.

S3.3 CV measurements on compound VI
Compound VI has shown an irreversible oxidation and an irreversible reduction. Changing the sweep rate did not result in observable cathodic current for the oxidation, nor anodic current for the reduction, indicating the irreversibility of the processes ( Figure S13). Interestingly, an irreversible reduction was observed for sweep rates higher than 100 mV/s at E p,c =-0.05 V (vs. Ag/AgCl).  The sample containing ferrocene displayed slightly unusual electrochemical behavior. The first cycle scanned from 0 V towards positive potentials showed a well-defined ferrocene/ferrocenium couple and an irreversible oxidation. Surprisingly, the 2 nd and 3 rd cycle S17 displayed an unusually high anodic current for the oxidation of ferrocene ( Figure S14). Also, the presence of the reference compound caused the appearance of small electrochemical signals with low currents, around the first oxidation (1.0 V) and reduction (-1.0 V). A voltammogram recorded in the presence and absence of ferrocene is shown on Figure S15.  An interesting observation was that after the experiments a red precipitate was found on the surface of the working electrode ( Figure S16). It is possible that the formation of this layer caused the unusual electrochemical behavior. S18 Figure S16. A red precipitate was observed on the surface of the working electrode after data acquisition. (Picture was taken by on of the co-authors (K.P.P.).) Electrochemical data extracted from the voltammograms is summarized in Table S5.
and  is a simple function to make sure that the first term is always greater or equal to 1. The delocalization indices of Eq. 1 were calculated using the overlaps between occupied molecular orbitals in the atomic basins generated by AIMAll program. 18  An antiaromatic (AA) counterclockwise ring-current can be observed throughout the pentalene ring system. The thiophene ring shows non-aromatic (NA) character.

PcT
A weak antiaromatic (WAA) counterclockwise ring-current can be observed throughout the pentalene ring system, while the thiophene ring shows NA character.

PbBT
AA ring-current can be observed in the pentalene ring system; the thiophene ring is NA; the benzene ring of the benzothiophene shows aromatic (A) character. BPbT Weak aromatic (WA) ring-current can be observed in the benzene ring of the benzopentalene moiety; AA ring-current in the pentalene ring system; NA character in the thiophene

BPcT
Aromatic ring-current in the benzene ring of the benzopentalene moiety; WAA ring-current in the pentalene ring system; NA character in the thiophene 6 BPbBT NA character in the benzene ring of the benzopentalene moiety; AA ringcurrent in the pentalene ring system; NA character in the thiophene; A ring-current in the benzene of the benzothiophene The pentalene moiety (Table S9) maintains higher antiaromaticity when it is fused to (benzo)thiophene through the "b" (2,3) bond of thiophene. Fusion through the "c" (3,4) bond of thiophene leads to alleviated antiaromaticity. These are in agreement with the bond order dependent antiaromaticity reported by Haley and co-workers. 20 The benzene ring that is fused to the pentalene subunit is weakening the antiaromatic character of the pentalene moiety.  to each other and to their conjugated cores (dashed line is the scan of the appropriate core).

S28
The prepared compounds and the corresponding conjugated cores have a similar NICS-XY scan profiles. The 'a' ring of V(PMP) has local aromaticity while the pentalene moiety of the molecule is antiaromatic. In compounds III and VI where the thiophene fusion is through its c (3,4) bond, the pentalene units shows low antiaromaticity/approaching non-aromaticity.
The NICS-XY scans and the ACID plots are in agreement regarding the (anti)aromatic characters of the studied molecules.

S4.2.3 Electronic aromaticity indices
To assess (anti)aromaticity by means of the electronic structure we used the multicenter index   compared to VI is also reflected in the extent of bond-length alternation across the thiophene/pentalene shared bond and the adjacent bonds within the pentalene framework.